In a smart device such as a smartphone or a tablet, a touch sensor is an indispensable component for allowing information to be input from a display unit, the touch sensor being provided in a manner overlaid on the display unit, for example.
In addition, from the viewpoint of operability, a smart device may have provided thereon, besides the main display unit, common function key buttons (so-called icons) such as a “home key” expressed as a quadrangular mark or the like, or a “return key” expressed as an arrow mark or the like. A common function key button is implemented using a planar light source in accordance with the pattern shape of the mark to be displayed in terms of improved visibility, and there is disclosed, for example, a configuration having an LED light guiding panel, which is a combination of an LED (Light Emitting Diode) and a light guiding panel, installed in the smart device (see, for example, Patent Literature 1 below).
In a smart device, also the aforementioned common function key button has provided thereon, in an overlaid manner, a touch sensor common to the main display unit implemented using a liquid crystal display device, for example.
In recent years, however, there has appeared an “in-cell” or “on-cell” liquid crystal display device with a built-in sensor electrode as a display for use in the main display unit. Accordingly, there is a strong demand to provide an independent touch detection function to the planar light source included in the common function key button.
There is disclosed a configuration of a planar light source provided with a touch detection function (see, for example, Patent Literature 2 below), in which a circuit board having for example a sensor electrode formed thereon is provided between a surface panel having an icon provided thereon and an LED light guiding panel, a through-hole is provided on a part of the circuit board where the icon is formed, and a layer of adhesive with a high dielectric constant is provided between the surface panel and the circuit board, whereby it is expected to improve the precision of capacitance detection by the sensor electrode.